This project attempts to characterize the contractile and metabolic functions of the skeletal muscle fiber types in the desert iguana Dipsosaurus dorsalis. The locomotory skeletal muscles of this lizard, as in many non-mammalian vertebrates, are composed of two twitch and one tonic fiber type. Fibers in Dipsosaurus muscles are segregated into oxidative and glycolytic regions, providing an excellent model system to study fiber specific functions. Three sets of experiments are proposed to test hypotheses of functions unique to each fiber type. Using the technique of glycogen depletion, experiments will examine the activity of fibers during high and low intensity exercise and during constant exercise intensity at either high (40 C) or low (25 C) body temperature. Data will be used to analyze fiber specific recruitment patterns based on exercise intensity and body temperature. A second set of experiments will analyze the isometric and isotonic contractile properties of single FOG and tonic fibers to evaluate the stimuli necessary for their activation, their potential contributions to tension generation during slow, deliberate versus high speed locomotion and their fatiguability relative to the FG fibers which predominate in Dipsosaurus. The third set of experiments will test the hypothesized metabolic specialization of vertebrate oxidative fiber types for rapid lactate catabolism and gluconeogenesis. Measurement of key gluconeogenic enzymes and an in vitro muscle incubation experiment will compare the gluconeogenic capacities of oxidative and glycolytic fiber types. Testing of the proposed hypotheses will carry the study of lower vertebrate muscle fiber types beyond the descriptive stage towards an understanding of when and how twitch and tonic fibers function during muscular activity, and provide information on the metabolic activity of these fibers both during and after activity. The proposed research uses muscle of a non-specialized lizard to address general questions of lower vertebrate muscle composition and function. The results of this research will help us to better understand the composition and function of lower vertebrate model systems frequently used to study contractile mechanisms and biomedical problems related to muscle and muscle disease.